In commercial practice, lithium metal is produced by the electrolysis of a molten mixture of lithium chloride serves in known manner to reduce the melting point of lithium chloride. Suitable electrolytic cells are, e.g., cells having no diaphragm. Such cells have a steel vessel, a steel cathode and a graphite anode and have no internal lining. The molten lithium metal accumulates on the surface of the molten salts and is skimmed from said surface by means of a skimming ladle or may be withdrawn by elevators. As chlorine gas is evolved and escapes from the cell, air will enter the cell so that the liquid metal may be oxidized and nitrided. Published European Patent Publication No. 107 521 discloses a process for the continuous production of lithium metal by an electrolysis of lithium chloride contained in a molten salt mixture an electrolytic cell comprising a cylindrical steel cathode, which has been inserted into the bottom of the cell, and a graphite anode, which is immersed into the molten material in the cell. In that known process, the molten salt mixture which contains lithium metal is withdrawn from the cell and the lithium metal is separated outside the cell. Because chlorine gas is evolved and the end of the cathode is formed like a venturi tube, a natural circulation is imparted to the molten material. A further reaction of lithium metal in the molten mixture is to be avoided.
Impurities of whatever kind are highly undesirable in the lithium metal if it is to be used in nuclear technology in the production of alloys and in lithium batteries.
For this reason it is known from U.S. Pat. No. 3,962,064 in the production of high purity lithium metal to perform the fused-salt electrolysis in an electrolytic cell which has no diaphragm and in which the lithium metal which has separated is collected on the surface of the electrolyte and the electrolyte level is raised so that the metal is forced out of the cell through a system of overflows and is conduced to a receiver. The receiver contains a protective gas atmosphere, in which the liquid lithium metal having a purity of 99.9% is cast to form ingots. That known apparatus has the disadvantage that the equipment is expensive and that air is used in the known process as a pressure fluid for raising the level of the electrolyte (and of the metal). Besides, the chlorine gas which is evolved is diluted with a large volume of air and is blown out of the cell together with this volume of air. This has the result that oxygen or air is inherently introduced into the system as an impurity, which is undesirable.